There is the need to measure environmental parameters to high accuracy with sensors that are light in weight, nonobtrusive, insensitive to electromagnetic interference and capable of withstanding extreme conditions including wide temperature extremes, large shocks and vibration. Applications for these sensors include the emerging area of smart structures where sensors are used to monitor environmental parameters that are used to augment manufacturing, perform nondestructive evaluation, implement health monitoring systems for structures and augment control systems. These structures need to be able to continuously monitor their condition in real time. They must be able to operate over extremely long times with low power. The sensors used to support these structures must be capable of accurately measuring small changes in environmental signals and still be able to maintain wide dynamic range capabilities.
Fiber optic gratings have been proposed for fiber optic sensors that have the potential to meet many of these requirements. The fiber gratings are constructed by doping the core of an optical fiber with material such as germanium. When exposed to light the index of refraction of the optical core of silica based fiber with appropriate core dopants have been observed to have a modified index of refraction. By using phase masks or interfering laser beams it has been demonstrated that it is possible to produce multiple variations in the index of refraction along the length of the fiber core producing an internal grating structure. Adjusting the spacing of the period during formation of the fiber grating changes its spectral transmission and reflection characteristics.
When a fiber grating is exposed to an environmental effect such as strain, temperature, pressure, or vibration the length of the optical fiber is changed and consequently the period of the fiber grating. As an example W. W. Morey reports in "Distributed Fiber Grating Sensors", Proceedings of the Seventh Optical Fiber Sensors Conference, p. 285-288, Sydney, Australia, December 1990 that typical temperature changes are 0.0043 nm/degree C. at 833 nm for Andrew PM fiber and 0.0074 nm/degree C. for Corning FlexCore fiber at 824 nm. When the fiber is strained the length of the fiber also changes. Bragg wavelength changes were measured by Morey to result in a shift of 5.2.times.10.sup.-4 nm per microstrain at 820 nm.
For many applications it is necessary to measure both temperature and strain simultaneously. E. Udd and T. E. Clark, in "Fiber Optic Grating Sensor Systems for Sensing Environmental Effects", U.S. Pat. No. 5,380,995 describe how using two overlaid fiber gratings at different wavelengths such as 1.3 and 1.5 microns may be used to measure two environmental parameters such as strain and temperature at a single point. Recently M. G. Xu, H. Geiger and J. P. Dakin, in "Multiplexed Point and Stepwise-Continuous Fiber Grating Based Sensors: Practical Sensor for Structural Monitoring?", Proceedings of SPIE, volume 2294, p. 69-80, 1994 have also demonstrated the simultaneous measurement of strain and temperature using 1.3 and 0.85 microns wavelengths and overlaid fiber gratings for point measurements. In order to make complete measurements of strain internal to a structure it is often necessary to measure all three strain components. R. M. Measures, D. Hogg, R. D. Turner, T. Valis and M. J. Giliberto, in "Structurally Integrated Fiber Optic Strain Rosette", Proceedings of SPIE, volume 986, p. 32-42, 1988 demonstrated a fiber optic strain rosette consisting of three separate fiber sensors. Because these fiber sensors are not collocated and because means to compensate for temperature fluctuation were not employed these rosettes have been of very limited use. There is a continuing need to measure other environmental effects such as transverse strain at a single point and to integrate such fiber grating sensors into practical and economical sensor systems that can be manufactured using available components.
In some situations transverse strain need not be measured and etalon based devices may be used what is needed are low cost, accurate means of measuring such environmental parameters as strain.